Cryptosporidium parvum is a significant human pathogen for which no chemotherapy has been shown to be efficacious. The number of cases of human cryptosporidiosis has increased dramatically in the last decade, particularly in the population of immunocompromised individuals. In the AIDS population, the mortality rate of Cryptosporidium infected individuals exceeds 60 percent. Hyperimmune anti-Cryptosporidium bovine colostrum, has shown a great deal of promise in limited human studies, providing therapeutic benefit to 7 of 8 cryptosporidiosis patients thus far treated. A possible impediment to the successful development of a Cryptosporidium bovine colostral immunoglobulin product is the potential for antigenic diversity among isolates or strains of this parasite, as there is a paucity of data addressing this issue. This proposal focuses on evaluating cross-reactivity and cross-neutralizing capacity against C. parvum isolates from calves and humans and from geographically diverse regions using anti-Cryptosporidium colostral immunoglobulin preparations with demonstrated protective properties. The long term objective is to discover strains that elicit broad cross-neutralizing antibodies for human therapeutic use.